As a semiconductor material, germanium (Ge) which is a chemical element of IV group has been used in a semiconductor device. However, presently, silicon is being widely utilized.
Originally, the germanium has a carrier mobility higher than that of the silicon. As such, the germanium has again attracted attention as a material for a highly-efficient solar cell or a material for the next generation of the silicon.
Concerned in handling the germanium is how to form a germanium thin film having a smooth surface on an underlying film such as an insulation film. In view of such a circumstance, a technique which heats a substrate having an insulation film formed on a surface thereof at a first temperature, supplies a diborane (B2H6) or a mixture gas of diborane/silane (SiH4) to the heated substrate, and then supplies a monogermane (GeH4)-containing gas was suggested. In the technique, it has found that the germanium film is formed after fully coating the surface of the insulation film with a boron-doped silicon layer, which allows the germanium to be uniformly grown.
In other techniques, it has found that, by depositing a silicon seed layer on a silicon dioxide (SiO2) substrate and continually depositing a germanium film on the silicon seed layer through a Chemical Vapor Deposition (CVD) method, a germanium film having a continuously smooth surface with a good flatness can be obtained.
As described above, in the suggested techniques, the boron-doped silicon layer or the silicon seed layer is interposed between the underlying film, such as the silicon dioxide, and the germanium film.
However, if a film other than the germanium film is interposed between the underlying film and the germanium film, the deterioration in the adhesion between the underlying film and the germanium film, the interface state occurrence, or the like can happen. That is, such a poor structure has negative effects on characteristics of semiconductor devices.
In addition, the suggested techniques may improve a surface smoothness of the germanium film. However, the techniques cannot make a thickness of the germanium film thinner while maintaining the germanium film to have a smooth surface with a good flatness and a good film thickness uniformity.